Doubly salient reluctance machines

ABSTRACT

The present invention relates to doubly salient reluctance machines (DSRMs) including switched reluctance machines, stepping motors and hybrid stepping motors. To improve the utilization of the machine windings and associated electronic circuits the windings are fully pitched across the stator poles, preferably with each stator slot containing conductors from a single winding only, and the power supply is switched to the windings to provide the required current sequence.

BACKGROUND TO THE INVENTION

The present invention relates to the general group of doubly salientreluctance machines (DSRMs), including switched reluctance machines(SRMs), also known as variable reluctance machines, stepping motors andhybrid stepping motors producing linear or rotary motion.

Doubly salient reluctance motors have received increasing attention overthe past few years, with a large number of publications reviewing theirrelevant merits with regard to other machine types. The DSRM has beenshown to produce a high specific output, despite rather poor utilisationof both the electrical and magnetic circuits, because of theintroduction of a magnetic gearing ratio, which arises from the doublysalient nature of the geometry. The magnetic circuit of the machine ispoorly utilised because each stator tooth can only be excited to producepositive torque for half of each rotation cycle.

It is to be understood that the term `saliency`, when applied toreluctance machines, implies magnetic saliency, which may or may notinvolve actual physical saliency.

A doubly salient reluctance machine has a stator and a rotor, both ofwhich exhibit saliency. Magnetic saliency is used here as generallyunderstood in the art, that is, a component of a reluctance machine(either its stator or rotor) is said to be salient if, in operation,changes in the reluctance of the magnetic circuit of the machine occurdue to the construction of that component as the relative position ofthe rotor and the stator changes during operation of the machine.

For example a conventional switched reluctance stepping motor is doublysalient since when a winding in energised and the rotor rotates towardsa new position, the main portion of the cross-sectional areas of theactive magnetic path in the both rotor and the stator increase and thereluctance of the magnetic circuit decreases due to the construction ofboth rotor and stator. In operation, energising different windingsselects different active magnetic circuits but the topography of aselected circuit varies as the rotor rotates.

A fuller description of switched reluctance motors and their principlesand applications can be found in the IEEE Industry Applications SocietyTutorial Course Publication "Switched Reluctance Drives" by J. M.Stephenson, S. R. MacMinn and J. R. Hendershot, Jr., as presented onOct. 12, 1990 at the IEEE IAS Conference In Seattle, Wash. The book"Stepping Motors: a guide to modern theory and practice" by P. P.Acarnley, published by Peter Peregrinus Ltd. on behalf of theInstitution of Electrical Engineers, provides an equally usefulpublication on stepping motors in general.

A related machine which Is not a stepping motor is the synchronousreluctance motor. Such a motor has saliency on the rotor only, thestator being similar to that of an induction motor. A device of thistype is disclosed in U.S. Pat. No. 5010267, which describes a variablespeed synchronous reluctance machine with a multiphase stator and arotor divided into segments which constitute flux guides. This machinehas a salient rotor, according to the definition of saliency as givenabove, but the stator has semi-closed slots and is not salient. Thetopography of the active magnetic path is determined by the flux guidesand as the rotor rotates the reluctance of this path changes due to theconstruction of the rotor only. The stator of the machine of U.S. Pat.No. 5010267 is fully pitched, a fairly common winding arrangement forsuch machines. Further mention of the significance of fully pitchedwindings will be made later on in this specification. The specificdesign of this machine is intended to reduce any effect of mutualinductance between phases as much as possible, as it is recognised thatin a machine of this sort mutual inductance will not produce torquewhich will add to that resulting from the changing self-inductance ofeach phase.

A further type of related machine is the hybrid stepping motor.Essentially, a permanent magnet provides a component of the magneticflux in this machine, with currents in at least one stator windingdirecting the flux along alternative paths. The interaction of the twomagnetic fields, one from the rotor magnet and one from the statorwindings, produces the torque on the rotor. The arrangement of statorpoles and rotor teeth and the selected excitation sequence determine themotion of the rotor. An introduction to and overview of these machinesis given in the above-mentioned book by P. P. Acarnley on pages 9 to 11.

Like the switched reluctance motor, this type of machine is also a DSRM.Once again, the stator poles can only be excited to produce torque forhalf of each rotation cycle, so the machine cannot be utilised to greatefficiency.

Another type of related machine is the so-called Vernier reluctancemotor,described in the Proceedings of the IEE, Volume 121, No. 9,September 1974 "Vernier Reluctance Motor" by K. C. MuKherji and A.Tustin. This machine has three phase distributed windings, arranged toproduce torque due to changing self inductance. Each phase cancontribute to positive torque production for a maximum of one half ofeach cycle.

Mention has already been made of fully pitched windings with relation tothe synchronous reluctance motor. The `pole pitch` of a reluctancemachine is defined as the peripheral distance between correspondingpoints on two consecutive simultaneously excited poles of opposite sign,whereas the `coil pitch` is defined as the distance between the twoactive conductors, or coil sides, of a coil. A fully pitched winding isone in which the ratio of the coil pitch to the pole pitch is 100%, inother words, the two are equal.

Fully pitched windings may be `concentrated` or `distributed`. In theformer, the peripheral distance between each coil side of a coil isequal to the pole pitch, and there will generally be one winding slotper phase per magnetic pole. In the latter, each winding is split into anumber of regions on each coil side and the peripheral distance betweensome of these opposed regions will not be the same as the pole pitch.

A salient stator in reluctance machines commonly carries a number ofevenly spaced projecting regions, or stator poles, between which thecoils are wound in slots. Furthermore, each stator pole may feature anumber of projecting teeth to act as flux guides at its extremity. Therotor itself may feature radially projecting portions which in operationdefine poles and have the effect of making the rotor `salient`.Alternatively, as in some synchronous reluctance machines, the rotorpoles may not be readily apparent to the eye. The rotor may have aplurality of salient teeth around its periphery to act as flux guides.How the poles and any teeth of the stator and rotor are arranged dependsof course on the precise type and design of machine.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the utilisation ofthe machine windings by changing the manner in which the machine iswound, so that more efficient operation is possible.

According to a first aspect of the invention there is provided areluctance machine comprising a stator and a rotor, each constructed tocause changes in the reluctance of the magnetic circuit as the relativeposition of the rotor and the stator changes during operation of themachine, the stator carrying conductors arranged and terminated to allowcurrents to flow around a plurality of loops each of which has at leasta pair of portions in which current flows in opposite directions withrespect to that direction which is normal to the direction of movementof the rotor to form magnetic poles and wherein, for each loop, eachportion carrying current in one direction is separated from each portioncarrying current in the opposite direction by a peripheral distanceequal to that separating adjacent magnetic poles of opposite sign.

According to a second aspect of the invention there is provided a doublysalient reluctance machine comprising a stator and a rotor, both withsalient poles, the stator carrying conductors arranged and terminated toallow currents to flow around a plurality of loops each of which has atleast a pair of portions in which current flows in opposite directionswith respect to that direction which is normal to the direction ofmovement of the rotor to form magnetic poles and wherein, for each loop,each portion carrying current in one direction is separated from eachportion carrying current in the opposite direction by a peripheraldistance equal to that separating adjacent magnetic poles of oppositesign.

According to a third aspect of the invention there is provided a doublysalient reluctance machine comprising a stator and a rotor both withsalient poles, the stator carrying conductors arranged and terminated toallow currents to flow along a plurality of loops, wherein theconductors are connected to form stator windings, each of whichcomprises a group of said loops and forms a current path such that, inoperation, substantial torque developed by the machine is due to changein mutual inductance between the paths as the rotor rotates.

Torque developed by a machine of the invention may be supplemented by,or supplemental to, torque due to self inductance of the windings. Thusmachines according to the invention may develop torque partly due tomutual inductance and partly due to self inductance.

The conductors may be arranged and terminated to allow unidirectionalcurrents only.

As a consequence of the construction of the stator and the rotor eachhave a number of salient poles, the number of stator poles being otherthan an integer multiple of the number of rotor poles.

Doubly salient reluctance machines according to the invention may bemotors or generators.

In operation, the windings are connected to supply means for supplying asequence of currents which produce a net unidirectional torque on therotor. The supply means usually comprises switching means connecting thewindings to a power supply and controlled to provide the requiredcurrent sequence.

An advantage of the invention is that a significant increase in torqueand efficiency is produced within a given frame size.

In a preferred embodiment of the present invention, conductors from asingle winding substantially fill the winding region in a slot betweenadjacent stator poles.

According to a further aspect of the invention there is provided ahybrid stepping motor comprising a stator with at least two windings anda rotor, wherein each winding is fully pitched.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 shows the principal components of a switched reluctance drive;

FIG. 2 is a cross-section of a prior art switched reluctance motor withsix stator poles and four rotor poles (a 6-4 SRM), showing windings fortwo stator poles only;

FIG. 3 shows the resultant magnetic flux pattern from the prior art 6-4SRM;

FIG. 4 is a cross-section of an SRM according to the present invention,showing two (of three) fully pitched windings;

FIG. 5 shows an SRM according to the present invention and illustrates afully pitched winding arrangement showing all the windings;

FIGS. 6 and 7 illustrate examples of possible conduction sequences forthe SRM of FIG. 5, to produce anti-clockwise rotation of the rotor;

FIG. 8 is a cross-section of an SRM according to the present inventionshowing all three fully pitched windings simultaneously excited;

FIG. 9 illustrates another possible conduction sequence for the SRM ofFIG. 5 to produce anti-clockwise rotation of the rotor with simultaneousexcitation of all three phases;

FIG. 10 shows the results of torque-angle tests for the conventionalmachine and for alternative conduction sequences of the machineaccording to the invention;

FIG. 11 is a long section through the axis of rotation of a conventionalhybrid stepping motor showing the rotor and stator assembly;

FIGS. 12 and 13 represent transverse cross sections through section X--Xand Y--Y respectively of the assembly of FIG. 11;

FIGS. 14 and 15 illustrate the excitation patterns of the windings ofthe assembly of FIG. 11 with respectively one and two phases excited;

FIG. 16 represents a transverse cross section of a hybrid stepping motoraccording to the invention; and

FIGS. 17 and 18 illustrate the excitation patterns of the windings ofthe motor of FIG. 13 with respectively one and two phases excited.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The invention will first be described with reference to its applicationto a switched reluctance machine.

In FIG. 1 the principal components of a switched reluctance drive areillustrated. In this application the SRM is in operation as a motor.

A d.c. supply is switched in sequence across the windings of a switchedreluctance motor 10 by a switching unit 11 under the control of anelectronic control unit 12. Switching is correctly synchronised to theangle of rotation of the motor 10 by using a rotor position encoder 13on the motor shaft to supply signals to the control electronics. In thisway each winding of the motor is excited in sequence for a part of thecycle of rotation. The motor speed can be set at the control unit 12.Further details of the principles and basic construction of SRMs aregiven in pages 4 to 7 of the IEEE IAS Conference Paper mentioned above.

in FIG. 2 a typical doubly salient switched reluctance machineillustrates the prior art, here with six stator poles (S₁ to S₆) andfour rotor poles (R₁ to R₄), that is, a 6-4 SRM. Both the stator androtor are laminated and each exciting coil is carried on a single statorpole, opposite coils being connected to produce a north and southpole-pair. Only one phase winding, formed by coils 20 and 21, is shownhere, to illustrate the excitation of an opposing pair of stator polesS₁ and S₄. In the rotor position shown the coils 20 and 21 when passingthe currents indicated conventionally provide positive reluctance torqueon the rotor teeth R₁ and R₃. This torque is developed by the tendencyfor the magnetic circuit to adopt a configuration of minimum reluctance,that is, for the rotor poles to move into line with the stator poles andto maximise the inductance of the excited coils. Note that the torque isindependent of the direction of current flow so that unidirectionalcurrents can be used, permitting a simplification of the electronicswitching circuits compared with those required for most other forms ofmotor.

FIG. 3 shows the magnetic flux diagram for the SRM of FIG. 2, againillustrating only one phase winding. In order to produce motoring torqueeach winding is switched on at a rotor position corresponding to lowself-inductance and off at a position of high self-inductance.Consequently each winding can only be utilised for a maximum of half ofeach rotation cycle, that is, it cannot be used for the period overwhich the self-inductance is falling.

Consider now the arrangement illustrated in FIG. 4 for a switchedreluctance machine according to an embodiment of the present invention.The windings are shown by reference numerals 22, 23, 24 and 25 and it isclear that in comparison to the arrangement of FIG. 2 twice the windingarea is available to achieve the same basic excitation pattern.

To achieve this excitation pattern, two windings are used, the windingseach being fully pitched across poles which, in operation, form adjacentexcited poles of opposite sign. The full winding area on either side ofeach stator pole is utilised to produce excitation in that pole. Onewinding is thus formed by groups of conductors 22 and 23, and another isformed by groups of conductors 24 and 25. With the correct switchingsequence the windings can be switched on to excite the rotor poles inthe sequence needed to provide unidirectional torque on the rotor.Examples of suitable currents are shown by the usual "cross" and "dot"symbols representing opposite directions of current flow.

in FIG. 5 the 6-4 SRM of FIG. 4 is shown and all the windings arerepresented. Each winding comprises in this case two groups ofconductors in opposite stator slots carrying current in oppositedirections. As in a conventionally wound 6-4 SRM, the power supply fromthe switching unit 11 is a three-phase supply, with groups of conductorsa+ and a- carrying a single phase A, and so on.

An example of a unipolar switching sequence which producesunidirectional, anti-clockwise torque on the rotor is given in FIG. 6.The angle θ of the rotor determines when each phase, or winding, isactive. In the rotor position illustrated in FIG. 5, correspondingapproximately to θ=45°, phases A and B are turned on and stator poles S₁and S₄ are therefore excited. Flux linkages with rotor teeth R₁ and R₃produce anti-clockwise torque on the rotor. At θ=60° the switching unit11 switches off phase A and switches on phase C, thus phases B and Conly are then active with stator poles S₂ and S₅ excited, and the rotorteeth R₂ and R₄ are urged into position. The switching sequencecontinues according to FIG. 6. Each conductor is used to produceexcitation in both neighbouring stator poles but for different periodsof the cycle, and thus unlike the conventional SRM winding arrangementeach phase is switched on for two thirds of each cycle, and throughoutthis period it is contributing to torque production.

An alternative sequence is shown in FIG. 7 and uses switching circuitswhich pass currents in alternate directions through the windings. A "+"in FIG. 7 indicates that current is passed in one direction while a "-"indicates current in the other direction.

In conventional switched reluctance machines the torque produced isdetermined directly from the area enclosed by the flux linkage-currentlocus. In the new modified winding arrangement the flux linking any onephase winding is no longer solely a function of position and its ownphase current. Coupling between phases results in the flux linkages alsodepending upon the other phase currents. Nevertheless the energyconverted to torque each time a phase conducts remains equal to theenclosed area of the locus for that phase. The self-inductance of anyone phase of a conventional SRM rises as rotor teeth come into alignmentwith the excited stator teeth and a torque is produced according to:##EQU1##

In general, where more than one phase may conduct at any one instantthis equation may be generalised. For example, in a three phase machine,this gives: ##EQU2## where i_(a), i_(b) and i_(c) are the instantaneousphase currents.

Ignoring saturation effects, as the self-inductance of each phase of aconventional SRM typically rises for a maximum of half of each rotationcycle, then this means that the phase can only be excited to producemotoring torque for a maximum of half of each such cycle. The newarrangement produces positive torque for considerably more time thanthis, and therefore must use some other mechanism. There is nowconsiderable coupling between phases, and therefore equation (2) isincomplete.

By considering the instantaneous voltages for each phase of this newwinding arrangement it can be shown that: ##EQU3##

Comparing equation (3) with equation (2) shows how where there iscoupling between phases then an additional set of terms contribute tothe torque, resulting from the changing mutual inductances between thevarious phases.

Assuming leakage and fringing fields to be negligible and iron paths tobe infinitely permeable then in the SRM of FIG. 5, the self-inductanceof each phase is directly proportional to the length of overlap betweenrotor and stator poles through which flux generated by the phase currentpasses. As one rotor pole comes into alignment then another one comesout of alignment, so that this length of overlap is constant andindependent of rotor position. The self-inductance of each phasetherefore does not vary with rotor position and is in fact equal to themaximum aligned phase inductance of a conventionally wound SRM. Hencethe first three terms on the right hand of equation (3) are zero. Such aconclusion is very significant in that is solely these very terms whichproduce the torque in a conventional SRM.

Using the same assumptions as above it can be shown that the mutualinductance M_(AB) between phases A and B is given by the followingequation: ##EQU4## where N=number of turns per phase

l_(a) =machine axial length

l_(ag) =air-gap length

1_(m) =tooth overlap dimension contributing to mutual inductance. (Thisdimension is illustrated by way of example in FIG. 4 and in this examplethe tooth overlap dimension is the difference between a length x, whichis the overlap of the poles R₂ and S₃, and a length y, which is theoverlap of the poles R₃ and S₄.)

μ_(O) = magnetic permeability of free space.

The simplified analysis above indicates that, in the 6-4 machine examplethe mutual inductance between phases is negative and constant for thirtydegrees of rotation; it then rises to an equal positive value during thenext thirty degrees, only to fall back to the negative value in anotherthirty degrees--hence completing the cycle. The magnitude of the maximumpositive and negative values of mutual inductance in this idealisedmachine are equal to the constant self-inductance of any one phase,minus the leakage reactance.

As shown above, the idealised machine according to the invention derivesits torque solely from changing mutual inductance. Machines with morethan three phases can be produced according to the invention. Althoughin such cases the idealised machine will not derive its torque solelyfrom mutual inductance terms, a substantial component of the torque willbe generated this way.

The instantaneous torque on the rotor when two phases, say a and b, areconducting is given by: ##EQU5##

Comparison of equation (5) with equation (1) shows that the rate ofchange of mutual inductance in equation (5) is twice that of the rate ofchange of self-inductance in equation (1). As the two phases are bothcontributing to the magnetomotive force (mmf), then for a giveninstantaneous phase current twice the mmf is available and thereforefour times the torque is produced, ignoring the effects of saturation,fringing and end winding losses. In a magnetically saturated machine thegains are substantially reduced, but remain significant.

The switching sequence of FIG. 6 uses two positive phase currents at atime when the mutual inductance between these phases is rising but thefalling portion of mutual inductance is not utilised. Whenbi-directional-current operation occurs, as with the switching sequenceof FIG. 7, then intervals of falling mutual inductance provide positivetorque since one of the phase currents is negative.

An alternative excitation pattern is one in which all three phasesconduct simultaneously, as illustrated in FIG. 8, which shows poles S₁and S₄ excited. This utilises both the rising and the falling portionsof mutual inductance. A greater MMF is present to produce positivetorque on one pair of rotor teeth, but a small MMF also results innegative torque on the other pair of rotor teeth. FIG. 9 depicts asuitable switching sequence to produce anti-clockwise rotation of therotor.

As an alternative to the 6-4 SRM a three phase 12-8 DSRM may beconstructed with six windings, each fully pitched, and machines with anypractical integer multiple of a 6-4 may be made according to theinvention. In operation of the 12-8 SRM the windings are excited toprovide two active pole-pairs at a time by supplying the windings inpairs, with the windings of each pair conducting for the same intervals.Twelve winding regions are provided and a fully pitched winding occupiesregions spaced by 90°, that is, the two portions of each winding arespaced by two regions occupied by other windings.

Tests were carried out to compare the alternative excitation sequencesdescribed above with a conventionally wound SRM. An existing 7.5 KWcommercial 12-8 SRM with a nominal rated torque of 48 Nm, was rewoundwith fully pitched windings. This machine was in a D132 frame size, witha core diameter of 210 mm and a stack length of 194 mm. Torquemeasurements were made using a commercial torque transducer with therotor locked at approximately one degree intervals between the alignedposition (0°) and the unaligned position (22.5°). Apart from therewinding operation, no modifications were needed to the machine. Thenumber of series turns per phase and the winding cross sectional areawere Kept identical to the conventionally wound version, but because ofthe increased end winding length the mass of copper increased by 44%from 8.34 Kg to 12.0 Kg. There was a corresponding increase in per phaseresistance from 0.797 ohms to 1.147 ohms at 20° C.

FIG. 10 illustrates the results, displaying torque against rotor anglefor excitation patterns A (unipolar conduction, two phases conducting ata time), B (bipolar conduction, two phases conducting at a time, C(bipolar conduction, all three phases conducting at a time, and D(conventionally wound SRM).

In this test, the peak current in the conventional machine was 15A andthe tests were performed on the basis of equal winding loss in themachine. It is clear from the results that with the winding arrangementaccording to the invention torque is substantially increased. Theconventional machine produced a peak torque of 48.2 Nm, excitation Aproduced a peak torque of 66.7 Nm (an increase of 38%), excitation B apeak of 67.8 Nm (an increase of 41%) and excitation C a peak of 76.8 Nm(an increase of 59%).

The invention has so far been illustrated with reference to a threephase SRM with 6-4 or 12-8 construction. However, it can be applied tomachines with other numbers of phases and different numbers of statorand rotor poles, depending on the intended application of the machine.Examples of applications of the present invention include 8-6 and 12-10SRMs, and also motors with poles which are divided into a number ofsalient regions adjacent to the rotor, but these examples are in no waylimiting.

Other forms of winding giving change in mutual inductance with rotorposition may also be used.

The principle of operation of stepping motors and hybrid stepping motorsis very similar to that of SRMs and thus the present invention alsorelates to doubly salient reluctance machines in the form of steppingmotors and hybrid stepping motors.

An embodiment of the invention in the form of a hybrid stepping motorwill now be described.

FIG. 11 shows a conventional hybrid stepping motor, although the figureis equally applicable to a motor according to the invention. A stator101 carrying stator windings 102 consists of two longitudinallyseparated sets of toothed poles 103,104 projecting radially inwardlyfrom a cylindrical back iron 105 mounted within a cylindrical housing(not shown). A motor shaft 106 carries the rotor 107, consisting of acylindrical permanent magnet 108 and two toothed cylindrical end caps109,110. Each toothed end cap corresponds in axial position with one ofthe sets of toothed stator poles and the two end caps are respectivelyin magnetic contact with opposite poles of the permanent magnet. A smallair gap 111 remains between the tips of the stator poles and the tips ofthe teeth of the rotor end caps. The stator 101 and the rotor end caps109,110 are both made from laminated soft iron.

A magnetic flux path is shown by dotted lines 112 leading from the northpole of the permanent magnet 108, through and radially outward fromrotor end cap 109, across the air gap 111 and through stator pole 103,axially along the stator back iron 105, radially inward through statorpole 104, across the air gap, and back through the rotor end cap 110 tothe south pole of the permanent magnet 108. The magnetic flux thereforehas to flow radially outwards from rotor end cap 109 and radiallyinwards towards rotor end cap 110.

The cross sections of FIGS. 12 and 13, which only apply to conventionalhybrid stepping motors, are taken respectively through planes X--X andY--Y of FIG. 11. The motor illustrated has 8 stator poles, numberedsequentially P1 to P8 in the figure, each pole having 5 uniformallyspaced teeth 120 and carrying part of a winding which extends across thecorresponding poles of both sets of rotor poles. The rotor end capscarry 50 uniformally spaced projecting radial teeth 121, the position ofthe teeth being angularly offset between the two rotor end caps as shownin the figures, such that in axial projection a tooth of one rotor endcap is located midway between two adjacent teeth of the other rotor endcap. The angular spacing of the teeth on a single stator pole issubstantially equal to that of the rotor teeth.

Two windings are provided in the assembly, each carrying one phase ofthe power supply A or B, and each winding is situated on four of theeight stator poles, winding A being situated on poles P1, P3, P5, andP7, and winding B being situated on poles P2, P4, P6 and P8 as shown inthe figures. Successive poles of each phase are wound in the oppositesense, such that, for example, excitation of winding A results in amagnetic field in one direction in poles P1 and P5 and in the oppositedirection in poles P3 and P7. This winding arrangement is represented inthe individual windings as depicted in FIGS. 12 and 13, the conventionbeing used whereby a+ and a-, for example, represent conductors carryingphase A in opposite directions. As a result of this winding arrangementa magnetic flux may develop in two adjacent stator poles of the samewinding, such as poles P1 and P3, this flux having the same phase butopposite polarity in the two poles when the stator winding is energised.

The operation of the conventional hybrid stepping motor will beexplained with reference to FIG. 14, which represents section X--X andshows the excitation of the windings when a single phase is operated. Ashaded winding in the figure represents excitation in the directionindicated by the conventional representation of `cross` and `dot`symbols.

The windings are used to encourage or discourage the flow of magneticflux through certain poles according to the rotor position required.With the excitation as shown in FIG. 14 the pole magnetic field will beradially outward in poles P1 and P5, whilst it will be radially inwardin poles P3 and P7. As a result the rotor will tend to align itself intothe position shown in FIG. 14, such that there is rotor teeth alignmentwith the teeth of stator poles P1 and PS, whilst at section Y--Y thealignment will be with poles P3 and P7. The torque is thereforedeveloped due to the changing magnetic flux linkage between stator androtor teeth.

The phase windings are excited in a sequence to produce rotor motion asdesired. If the excitation of A is removed and B is excited with reversecurrent then the stator and rotor teeth will tend to align under polesP4 and P8 of section X--X and poles P2 and P6 of section Y--Y, thusmoving the rotor through one step in an anti-clockwise direction. Toproduce continuous rotation in this direction, the sequence is continuedas shown in the following table, which depicts the switching sequenceand the resulting alignments.

    ______________________________________                                        Sequence for anti-clockwise rotation                                                    Pole field  Pole field                                                        at X--X     at Y--Y  Rotor position                                 Excitation                                                                              (outwards)  (inwards)                                                                              (Degrees)                                      ______________________________________                                        A+        P1,P5       P3,P7    0                                              B-        P4,P8       P2,P6    1.8                                            A-        P3,P7       P1,P5    3.6                                            B+        P2,P6       P4,P8    5.4                                            A+        P1,P5       P3,P7    7.2                                            ______________________________________                                    

The length of each step is 360/4 n degrees, where n is the number ofrotor teeth, since a complete excitation cycle of four steps results inalignment of teeth under the same stator poles. Thus in the illustratedexample the motor has a step length of 360/200, or 1.8 degrees, and itis this small step length which allows very high resolution in angularpositioning of such hybrid motors.

For clockwise rotation the switching sequence will be different, theexcitation of the phases being in the order A+,B+,A-,B-,A+, . . .

Torque can be improved by exciting more than one phase at a singleinstant, and this is represented for the motor of the example in FIG.15, which shows the excitation at section X--X. The excitation sequencefor anti-clockwise rotation will be as follows:

(A+/B+),(A+/B-),(A-/B-),(A-/B+),(A+/B+), . . . This arrangement ishowever very inefficient, due to the fact that one half of the slotshave no MMF in them since they carry two components of current inopposite directions (shown unshaded in FIG. 15). Hence they still haveconsiderable loss, but with little benefit in terms of torqueproduction.

Consider now the arrangement illustrated in FIG. 16 for a hybridstepping motor according to an embodiment of the present invention. Twowindings--phases A and B--are again used. This time the windings arefully pitched across the stator poles such that each winding spans twostator poles.

The full winding area on either side of each stator pole is utilised andwith the correct switching sequence the windings can be switched on toexcite the rotor poles in the sequence needed to provide the desiredmotion of the rotor. The appropriate sequence for anti-clockwiserotation will be B+,A+,B-,A-,B+, . . . , and the motor is shown in theposition corresponding to the first step of this sequence in FIG. 17,the figure illustrating the situation at section X--X. This arrangementwill generate the same torque as that of the conventional hybrid motorshown in FIG. 15, but will suffer from only one half of the loss in theactive region of the winding (neglecting end losses).

The basic MMF pattern with one phase conducting can be reproduced withtwo phases conducting in the machine according to the invention, asshown in FIG. 18. There is now twice the area of copper in which toproduce the same MMF pattern, so that greater torque will result for agiven copper loss. Conversely, for the same torque there will be a muchreduced copper loss. The appropriate switching sequence in this casewill be as follows:

(A+/B+),(A+/B-),(A-/B-),(A-/B+),(A+/B+), . . .

it is to be noted that unlike the fully pitched SRM, the machine doesnot derive its torque from mutual inductance due to interaction betweenstator phases.

The invention has been illustrated with reference to a two phase hybridstepping motor with 8 stator poles, but this example is in no wayrestrictive to the scope of the invention. The invention can be appliedto a machine having stators with other numbers of poles and using morethan two phases. Similarly the number of teeth carried on the rotor andon the stator poles can be varied, depending on the intended applicationof the machine.

It is also to be realised that these machines are generally designedwith a number of basic motor units (stacks) along the axial length ofthe machine, the number of stacks also being selected as appropriate tothe application.

For operation of the hybrid stepping motor, not only the stator androtor assembly as described above are required, but also means forproviding multi-phase currents to the windings and the means toaccomplish the desired switching. A rotor position encoder, typically anopto-electronic device operated from the rotor shaft, may be included tosupply signals to control electronics in order to ensure correctsynchronisation of the switching.

The foregoing description concerns applications of DSRMs as motors.However, DSRMs according to the invention may also be generators. In theabsence of permanent magnetic poles, the windings require excitationfrom a power supply by way of switching circuits. The rotor is driven tooppose the torque developed by the windings and electrical power flowsfrom the machine into the power supply.

It will be appreciated in this specification and the accompanying claimsthat the terms rotor and stator also apply to linear machines, where therotor and the stator are the moving and the stationary members,respectively.

While the invention has been described in conjunction with specificembodiments thereof, it is intended to embrace all other embodimentsthat fall within the spirit and scope of the appended claims.

I claim:
 1. A reluctance machine comprising:a stator having a core made from a highly permeable magnetic material; and a rotor, freely rotatable to be drawn into a position of alignment with the stator to minimize the reluctance at any point in time, both the stator and said rotor being constructed to cause changes in the reluctance of the magnetic circuit as the relative position of said rotor and said stator changes during operation of the machine, the stator carrying conductors arranged and terminated to allow currents to flow around a plurality of loops, the conductor loops capable of being energized from a multiphase current supply such that they may be sequentially energized for displacement of said rotor between successive positions of minimum reluctance, each of which loops has at least a pair of portions in which current flows in opposite directions with respect to that direction which is normal to the direction of movement of the rotor to form temporary magnetic poles within the stator, said magnetic poles being arranged in regularly spaced opposed pairs of poles, and wherein, for each loop, each portion of the loop carrying current in one direction is separated from each portion of the loop carrying current in the opposite direction by a peripheral distance equal to that separating adjacent magnetic poles of opposite sign.
 2. A machine according to claim 1 wherein the conductors are connected to form stator windings, each of which comprises a group of said loops and forms a current path such that, in operation, substantial torque developed by the machine is due to change in mutual inductance between the paths as the rotor rotates.
 3. A machine according to claim 1 or 2 wherein said conductors are arranged and terminated to allow unidirectional currents only.
 4. A machine according to claim 1 wherein as a consequence of the construction of the stator and the rotor each have a number of salient poles, the number of stator poles being other than an integer multiple of the number of rotor poles.
 5. A machine according to claim 1 in which the conductors are located in stator slots and each slot contains conductors from a single winding only.
 6. A machine according to claim 1 wherein the rotor comprises a permanent magnet polarised in an axial direction and is so arranged that for every one of a plurality of rotor positions there is a respective preferential flux path between the rotor and the stator poles.
 7. A machine according to claim 6 wherein the rotor comprises two generally circular low-reluctance members arranged on the rotor axis one at each end of the permanent magnet, each said member having a plurality of teeth equally spaced around its circumference, and the stator comprises a plurality of saliently shaped poles each carrying a plurality of teeth adjacent to the rotor.
 8. A machine according to claim 7 comprising two windings, eight stator poles and fifty rotor teeth.
 9. A machine according to claim 1 three windings, six stator poles and four rotor poles, or any integer multiple of these numbers.
 10. A machine according to claim 1 in combination with means for supplying multi-phase currents to the conductors.
 11. A combination according to claim 10 wherein, in operation, currents are supplied to the windings in a sequence which produces a net unidirectional torque on the rotor. 